Introduction

EM Electronics is a known name in very low signal voltage and current measurement applications.

Overview

There is overview page on EM site covering A10 specifications and features

Exterior

Interior

Interface module design, connection to DMM

Sensitive amplifier, such as EM A10 require special care in every aspect of application and operation. Accurate measuring nanovolt-level signals is more state of art, rather than usual engineering toss-all-parts together and power it on. All things matter, there are no insignificant details.

Good example of usage scenario and setup is covered in Keithley’s instruction manual for Model 1801. To be precise, Model 1801 is older version of EM A10 amplifier, with few extra things in it, such as gain switching and selectable filtering caps. Original Model 1801 kit (consists of Model 1801 preamplifier head, RJ50 interface cable and addon power supply card for K2001/K2002 DMMs) is long obsolete, and not possible to buy from Keithley or distributors, so we have no choice left but to design very own interface board to connect A10.

Example gain settings and input/output voltages

Range at input

Input signal

GAIN

Output

RFB

RGAIN

Vout

1 mV

2.00-3

1000

2.0000

1 Ω

999 Ω

2.000000 V

100 uV

2.00-4

10000

2.0000

1 Ω

9999.00 Ω

2.000000 V

10 uV

2.00-5

100000

2.0000

1 Ω

99999.0 Ω

2.000000 V

1 uV

2.00-6

100000

2.00-1

1 Ω

99999.0 Ω

200.0000 mV

100 nV

2.00-7

100000

2.00-2

1 Ω

99999.0 Ω

20.0000 mV

10 nV

2.00-8

100000

2.00-3

1 Ω

99999.0 Ω

2.0000 mV

1 nV

2.00-9

1000000

2.00-4

1 Ω

999999.0 Ω

2.0000 mV

100 pV

2.00-10

1000000

2.00-5

1 Ω

999999.0 Ω

0.2000 mV

Of course it’s easier to just power A10 amplifier with ±8.5VDC supply and connect it’s output to your favorite DMM, but we were interested in making nice usable kit, since I have multiple Keithley Model 2001 and Model 2002 DMMs. DMM firmware (Model 2001 must have B-version firmware to support preamp) already have support for 1801 preamp, and show proper ranges information.

Module specification

After review of existing Keithley 1801 and Model 2182, 182 nanovoltmeters specs it was decided to have next key features in this design:

Worth to note, that Keithley original transformers used in Model 1801 power supply card are possible to order, but price for them is extraorbitant, around 600 $USD. So while our design still support their use, second plan B option is also made to support usual push-pull converter using market transformer Eaton Bussmann CTX02-13665 and Linear LTC3439EME push-pull converter. Both boards are referenced in further text as XA1 for discrete push-pull with Keithley transformers and XA2 with LTC3439 option.

Schematics blocks

Original manual from 1801 preamp have schematics for power supply module, so we can use basic idea and design around it, using modern quality components for SMT and circuit design. We did contact Keithley in attempt to buy Model 1801, but got feedback that unit is obsolete and not available anymore. Also few extra features are added, not present in original design. It would be clear to note, that module and design discussed below is not a clone or simple copy of original 1801.

This part resembles original Keithley 1801 module card, as we want to be compatible with firmware controls.

Calibration constants and data is stored in I2C EEPROM, in our case 24C02 or bigger device. Optional resistors are available to customize address. I2C address also routed to DIP-switch.

Power supervision and reset generation, handled by TL7705, similar to Keithley circuitry here.

Isolated side is simply driving switches at head, with pull-up/pull-downs to isolated &plusmn8.5VDC supplies.

Keithley isolation transformers designed for push-pull topology, so we used 74HC74 DFF-based oscillator for 9.6kHz, with further divider by 2 to get differential complimentary drive at 4.8kHz for pair of N-channel MOSFETs. Stable capacitors 82nF and 988 Ω resistors being used to provide necessary stability and accuracy.

C30 acting as bulk capacitor to provide local transient supply and C54 is optional damper capacitor.

Positive isolated +5VDC for K1801 head logic supply generated by very low noise Linear LT3042 LDO in 10-DFN package. This LDO with current reference is adjustable in wide range by single resistor R45, and able to provide up to 200mADC of current with only 0.8µV noise. This regulator not needed for EM A10 head.

Negative isolated -5VDC for K1801 head logic supply is generated by similar low noise Linear LT3090 LDO. This device with current reference is adjustable in range by single resistor R47, and configured for maximum current up to 200mADC via R48. This regulator not needed for EM A10 head.

Main isolated low noise ±8.5VDC supply for analog part of head is generated from very low noise Linear LDOs, LT3042 and LT3090. Circuitry is same as ±5VDC supplies. If battery power instead of push-pull transformer converter used, this supplies provide input power for ±5VDC LDOs, otherwise everything isolated.

Output Molex connector is NOT pin-pin compatible to Keithley 1801 module, so we use own cable pinout to connect with nanovolt amplifier head. Output signal from amplifier routed to pin 3, which is shielded by low-impedance guarded ground to provide best noise isolation. Protection TVS-diodes D26,D27 near connector provide additional protection from overvoltage/reverse polarity for preamplifier head. Output is quality SMA port with copper terminals to minimize EMF and provide coax output.

STEP 1. Clean the input terminals, then connect the low thermal shorting strap to the EM A10 INPUTS terminals. I used copper single-strand from CAT5 Ethernet cable.STEP 2. Put preamplifier module in the thermal isolation enclosure. Remove the lid only while making adjustments.STEP 3. Turn on the Model 2001 power, and make sure the preamplifier is enabled. Select 20μV DC range, and allow a two-hour warm-up period before proceeding with the offset
adjustments.STEP 4. Adjust the voltage offset control (V ZERO) for a Model 2001 front panel reading of 0μV ±0.001μV.STEP 5. Enable the REL mode to null any remaining offset.STEP 6. Disconnect the low-thermal shorting strap from the INPUTS terminals.STEP 7. Connect the 10kΩ low thermal resistor to the INPUTS terminals, (For best results, remove the lead plating before use.) STEP 8. Allow a 15 minutes warm-up period before proceeding with the current offset adjustments.STEP 9. Adjust the current offset adjustments (I ZERO) for a Model 2001 front panel reading of 0μV ±0.1μV.STEP 10. Disconnect resistor. Offset adjustments are complete, now we can start normal gain calibration.